fix(core): detect path from decorator for each class only once

[Ruby184]

Fix for bun path detection

Related discussion here: #5496
In this commit you changed the condition because of coverage. This is incorrect because it guards against going out of stack array bounds - I have added the test for this case. Also, I removed no longer needed condition.

Performance

Before the changes, the path was detected for every used decorator on the class and overwritten. This is not needed because the path used as a key should be stable and should not change for a given class. So I introduced the new symbol MetadataStorage.PATH_SYMBOL, which will hold the path and check if it exists for a given class using Object.hasOwn (this also works for class inheritance so the path is not taken from the base class but defined for each child).

Moreover, as a side effect, this also allows defining a path on the class by the developer (maybe in cases like when Bun was not working), but it should not be needed and used only as a workaround:

import { Entity, MetadataStorage } from '@mikro-orm/core';

@Entity()
class User {
  static [MetadataStorage.PATH_SYMBOL] = import.meta.filename // or __filename for commonjs

  @PrimaryKey()
  id!: number;

  @Property()
  username!: string;
}

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.74%. Comparing base (271ff7d) to head (9546e07).
Report is 6 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5545   +/-   ##
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  Coverage   99.74%   99.74%           
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  Files         260      260           
  Lines       17972    17976    +4     
  Branches     4370     4372    +2     
=======================================
+ Hits        17926    17931    +5     
+ Misses         46       45    -1     

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[B4nan]

Moreover, as a side effect, this also allows defining a path on the class by the developer

This is great, I've been thinking about introducing this for ages. We should document this somewhere.

[Ruby184]

This is great, I've been thinking about introducing this for ages. We should document this somewhere.

Sure, but there is a caveat. I think people will expect to define the path in the base class and inherit it. However, they need to copy-paste the code for each class, which is slightly counter-intuitive. This comes from the fact that import.meta.filename or __filename is scoped to a module (file), and we expect the path to be defined for each class because it usually differs. It's not worth making logic to detect if we can inherit it from the parent, maybe if it is defined as a getter, but we can document that. I am open to suggestions.

[B4nan]

Yes, that is surely something to document. I don't think it makes sense to try to validate this, as having the entities in a single file is still a valid case.

[B4nan]

I guess this could go to https://mikro-orm.io/docs/metadata-providers.

Also, do you think this should be a perf commit? Does it have a measurable difference? Maybe a fix would be better, or even a feat since we are adding a new feature that will have its documentation.

[B4nan]

Btw does this help with the production build issue in bun mentioned in that discussion?

[B4nan]

I'll merge this in as a fix, as I don't want to hold it for no reason, but I would still appreciate response to my previous comment as well as a PR mentioning this in the docs somewhere.